The goal of this project is to reveal distinct roles played by tumor necrosis factor receptor (TNFR)-associated factor (TRAF)3 in T lymphocytes. During the >15 years since its discovery, the number of receptors for which TRAF3 plays important roles has grown considerably, including members of the TNFR-superfamily (TNFR-SF), certain cytokine receptors, innate immune receptors, and most recently, reported by our laboratory, the T cell antigen receptor (TCR) complex. Mutations leading to deficiencies in TRAF3 levels and/or function have been linked to human disorders as diverse as inadequate responses to viral pathogens and B cell malignancies. This makes TRAF3, and the signaling pathways it regulates, attractive targets for therapeutic manipulation. We now appreciate that the roles played by TRAF3 in immune cells can be quite distinct for different cell types, as well as for different receptors expressed by individual cell types. This highlights the importance of addressing the knowledge gap that is our long-term goal: understanding the mechanisms and biological consequences of TRAF3-mediated regulation of immune cell function. Such understanding cannot be gained only by analyzing TRAF3 in non-immune cell types, and the functions of TRAF3 in one cell type cannot be extrapolated to another. The objective of the proposed project is to build upon our recent novel findings to understand how TRAF3 contributes to T lymphocyte biology and functions. Our central hypothesis is that TRAF3 plays important roles in T cell biology by regulating both the strength of signaling by the TCR complex. During the proposed project period, we will pursue the following Specific Aims: (1) Determine how TRAF3 regulates TCR complex-mediated biochemical signals. (2) Investigate how TRAF3 participates in the immunological synapse. (3) Define how TRAF3 regulates iNKT cell development and function, via regulation of TCR signal strength. The expected outcomes of the proposed study are a detailed understanding of the different ways in which TRAF3 regulates function of T lymphocytes. The knowledge will have an important impact on both basic understanding of, and the ability to knowledgeably manipulate T lymphocyte populations and functions.